The Lake’s iconic transparency and stunningly blue waters are often the first thing that comes to mind when people think of the Tahoe Region. The lake’s clarity has been regularly measured since 1968, when UC Davis first started lowering a Secchi disk into the lake, establishing one of the longest, unbroken clarity measurement records in the world. The Secchi depth measurements are perhaps the best known of all the indictors of the Region’s environmental health. Between 1968 and 2000, the clarity of the lake regularly declined, but the long term decline in clarity observed in the end of the 20^th century appears to have been halted about15 years ago. While this is an encouraging trend, there is still much work to be done, and is but one of many measures of the health of Region’s aquatic systems.

The health of the aquatic system is assessed with respect to six threshold standards categories: 1) Lake Tahoe pelagic (deep) waters, 2) Lake Tahoe littoral (nearshore) waters, 3) tributaries, 4) surface runoffs, 5) groundwater, and 6) other lakes (i.e., lakes other than Lake Tahoe). Fine sediment particles (< 16µm) and nutrients that support algal growth (nitrogen and phosphorus) are the primary pollutants of concern in the Region because of the negative impact on transparency (Lahontan & NDEP, 2010a) and, in the case of nutrients, the blueness of the lake (Watanabe et al., 2016). Additionally, many components of the aquatic system are thought to be adversely affected by these pollutants (Reuter et al., 2009).

The Bi-State Compact requires the Regional Plan to provide for the attainment and maintenance of federal, state, and local water quality standards. Resolution 82-11 sets out numerical standards, management standards, and policy statements for water quality. Some of these threshold standards are referenced to state standards. In other cases, Resolution 82-11 sets targets based on  reference conditions related to specific periods, and can be found in the Study Report for the Establishment of Environmental Threshold Carrying Capacities (TRPA, 1982). The value statements were developed in 1982, that guided the development of threshold standards for water quality were:

• Attain levels of water quality in the lakes and streams within the basin suitable to maintain the identified beneficial uses of Lake Tahoe.
• Restrict algal productivity (rate of growth) to levels that do not impair beneficial uses or deteriorate existing water quality conditions in the Lake Tahoe basin.
• Prevent degradation of the water quality of Lake Tahoe and its tributaries to preserve the Lake for future generations.
• Restore all watersheds in the basin so that they respond to runoff in a natural hydrologic function.

Prior to the arrival of European settlers, fire, floods, and other natural disturbances (e.g., earthquakes, landslides, or avalanches) were the major drivers of pollutants like fine sediments and nutrients entering the lake. However, these were likely episodic in nature, with potentially substantial intervening periods between major events. More regular, low-intensity fires and a mature forest likely translated into low-nutrient stores on the forest floor. These were the watershed conditions that supported an ultraoligotrophic Lake Tahoe: a lake with a sustained level of exceptional water clarity (greater than or equal to 30 meters), a lake receiving low inputs of nutrients and therefore supporting low levels of primary productivity, and a lake containing a relatively simple food web that may have substantially relied on the recycling of nutrients and carbon, rather than new inputs from the surrounding watershed.

Urbanization and development altered the natural hydrologic regimes of many of the catchments in the Tahoe Region. Studies completed as part of the Lake Tahoe Total Maximum Daily Load (TMDL) show that urban areas are the primary source of fine sediment (the pollutant known to impact lake clarity) (Lahontan & NDEP, 2010a, 2010b). Much of the urban development has occurred along the edge of Lake Tahoe, meaning that in many cases, there is little or no buffer between the source of pollution and the Lake. The concentration of development also represents an opportunity for managers in the Region to mitigate impacts. For example, the Regional Transportation Plan focuses on ensuring that compact town centers are well served by transit, pedestrian and bicycle infrastructure, thus reducing reliance on cars, resulting less pollutant load reaching the lake. Reducing reliance on automobile is essential for addressing atmospheric deposition of nitrogen into the lake, which accounts for more than half of nitrogen loading (Lahontan & NDEP, 2010a, 2010b). 

The nearshore of Lake Tahoe is an increasingly important focus for managers in the Region. It is the portion of the lake that visitors and residents most often interact with, and the presence of invasive species (e.g. Eurasian watermilfoil and curlyleaf pondweed) and anecdotal reports of change in nearshore conditions have heightened awareness of the nearshore. In 2012, the TRPA Governing Board adopted two new standards related to the nearshore environment, to address attached algae (periphyton) and aquatic invasive species. In October 2013, the Desert Research Institute, University of California at Davis, and the University of Nevada at Reno released the Lake Tahoe Nearshore Evaluation and Monitoring Framework Report (Heyvaert et al., 2013a). The report presents a conceptual understanding of nearshore environmental processes, highlights the heterogeneous nature of the nearshore, identifies deficiencies in the data available to characterize the environmental status of the Lake Tahoe nearshore, and proposed a set of monitoring metrics. The report also indicated that the actions implemented by partners in the Region to improve pelagic water quality are likely to benefit nearshore conditions. Pilot monitoring efforts were initiated in 2014 and 2015 to address specific data gaps identified in the report. The number of parameters of interest in the nearshore (including clarity, metaphyton, periphyton, toxins, and others) and spatial variability around the lake (conditions in one location are not necessarily indicative of conditions in other locations) complicate management of the nearshore. Building upon the report and recent monitoring efforts, management partners in the Region, including representatives from U.S. EPA, Lahontan, NDEP and TRPA, are developing a nearshore resource allocation plan (NRAP). The NRAP will be a comprehensive framework for allocating resources to enhance our understanding of the nearshore environment and more effectively targeting management actions to preserve the resource for future generations. A draft version of the NRAP is expected to be available in late 2016. 

This chapter presents an evaluation of the water quality conditions and trends for the Region’s aquatic system relative adopted standards (Table 4-1). However, the health of the Region’s aquatic system is intimately linked to many of the components of the terrestrial system that are evaluated in other chapters of the report. The extent of impervious surfaces (soils chapter), the status vegetation and riparian areas (vegetation chapter), the condition of Region’s streams (fisheries chapter), and atmospheric deposition of nitrogen (air quality chapter) all strongly influence the pollutant load reaching the Lake. Although these factors are evaluated in other chapters of this report, they provide the context necessary to interpret the findings of this chapter and guide management in the Region.